Recent evidence suggests that progesterone is required for ovulation, luteinization, and the maintenance of luteal structure and function in primates. Progesterone action is mediated by intracellular progesterone receptors (PRs), and PRs are detectable by immunocytochemistry in the monkey corpus luteum. However, changes in total luteal PR and PR isoform expression have not been quantitated in the corpus luteum during its lifespan in the menstrual cycle. A western blotting technique for monkey PRs was developed to identify and quantify PR isoforms in the macaque corpus luteum throughout the luteal phase of the natural cycle. Several antibodies generated against the human PR recognized two bands of consistent sizes in monkey tissues, and these bands comigrated with PR-A and PR-B from human T47D cells. Taken together, these data suggest that the two proteins identified are macaque PR-A and PR-B. Predicted sizes of monkey PR-A and PR-B were approximately 90 and 120 kDa, respectively. PRs were detected in a variety of macaque tissues, including the endometrium, whole ovary, and decidua, but not in spleen, which is PR-negative by other techniques. Whereas PR-A was the predominant isoform observed in endometrium and decidua, PR-B predominated in whole ovary and in the corpus luteum. In luteal tissue, PR-A levels decreased (p<0.05) over the course of the luteal phase while PR-B levels were unchanged. Hence the ratio of PR-B to PR-A (PR-B/PR-A) increased (p<0.05) from early to very late luteal phase. Since PR-B/PR-A can alter gene expression in response to progestins and antiprogestins in vitro, the temporal changes in PR-B/PR-A in the monkey corpus luteum may contribute to functional differences in luteal responses to progesterone and other steroids in vivo.